In many situations it is either desirable or necessary to remove sludge and other solids settling in or suspending in liquid in a liquid reservoir. For example, in settling ponds such as settling ponds used in piggeries, dairies, poultry farms and animal feed lots, in sewerage settling ponds, in mining slurry ponds or other ponds, there tends to be an increase in solid matter over a period of time in the pond water. As a result of build up of this solid matter, it is often necessary to empty the pond at regular intervals. This creates an environmental problem in liquid disposal. Further after a pond is drained, heavy machinery is usually required to be used at considerable cost to remove and spread the solid matter which has settled as sludge on the base of the pond. The pond is then required to be refilled necessitating use of a large quantity of water. It would be desirable therefore to be able to efficiently remove sludge or other solid materials from ponds without emptying the pond.